Not applicable
1. Field of the Invention
This invention relates in general to liquid oxygen tanks and feed lines of composite material construction, and more specifically to oxygen tanks and feed lines of hybrid composites construction.
2. General Background
The development of polymer composite liquid oxygen tanks is a critical step in creating the next generation of launch vehicles. Future reusable launch vehicles need to minimize the gross liftoff weight (GLOW) by reducing the dry mass fraction. The (dry) mass fraction is the weight of the launch vehicle without fuel divided by the weight of the vehicle with fuel. FIG. 1 is graph showing the effect of mass fraction on GLOW. Indicated on the graph are the RLV mass fraction target region as well as the mass fraction of the RLV without the weight reduction that composites could provide. It is clear that composite tanks are critical to enable future launch vehicles to meet required mass fractions.
The required mass fraction is possible due to the reduction of weight that composite materials can provide. Traditional oxygen tanks are usually made from metals. The space shuttle external tank (ET) has historically been made from 2219 aluminum and more recently 2195 aluminum/lithium alloy. FIG. 2 shows a comparison between these two aluminum alloys and a typical composite material for a liquid oxygen tank for a launch vehicle. The chart shows that a composite tank provides up to 41% and 28% weight savings when compared to 2219 and 2195 aluminum alloy tanks, respectively.
Although a composite liquid oxygen, (LOX) tank makes the RLV mass fraction feasible, a liquid oxygen tank must be compatible with oxygen. The ASTM definition for oxygen compatibility is the xe2x80x9cability of a substance to coexist with both oxygen and a potential source(s) of ignition within the acceptable risk parameter of the user.xe2x80x9d It is imperative that materials are selected that will resist any type of detrimental, combustible reaction when exposed to usage environments. Typically, non-metallic materials are not used in these applications because most are easily ignited in the presence of oxygen. However, there are some polymeric materials with inert chemistries that may be used for this application and resist ignition. These chemistries were evaluated by fabricating coupons and testing them with various ignition mechanisms in the presence of liquid and gaseous oxygen. The testing performed reflected situations in launch vehicles that could be potential sources of ignition in composite. These tests included modified ambient pressure mechanical impact, pressurized mechanical impact, puncture, puncture of damaged, oxygen-soaked samples, friction, and pyrotechnic shock.
Some composite materials that have excellent resistance to ignition may not have the mechanical or physical properties required to make a feasible composite tank. Similarly, composite materials which have excellent mechanical and physical properties may not be resistant to ignition in the presence of oxygen. Therefore, a hybrid composite may be an ideal solution. A hybrid composite tank would have inner plies of an oxygen compatible composite, while the outer plies of the tank would be made from materials with better structural properties.
Applications for hybrid tanks include liquid oxygen tanks for future launch vehicles, such as the Lockheed Martin Reusable Launch Vehicle (RLV). They could also potentially be used in other aerospace applications, including but not limited to, RFP (rocket fuel propellant) tanks and crew vehicle cabins. Other industries that may be interested in composite oxygen tanks include the air handling and medical industries. The ability to resist ignition may also be useful in chemical storage tanks and NGV (natural gas vehicle) tanks.
The following U.S. Patents are incorporated herein by reference: U.S. Pat. Nos. 5,056,367; 5,251,487; 5,380,768; 5,403,537; 5,419,139; and all references cited in those patents.
The following international applications published under the PCT are incorporated herein by reference: International Publication Nos. WO 97/18081 and WO 97/28401 and all references cited in those publications.
A fiber-reinforced composite is defined as a material consisting of fibers of high strength and modulus embedded in or bonded to a matrix with distinct interfaces or boundaries between them. In this form, both fibers and matrix retain their physical and chemical identities, yet they produce a combination of properties that cannot be produced by either constituent alone. In general, fibers are the principal load carrying members, while the surrounding matrix keeps them in desired location and orientation, transfers loads between fibers, and protects the fibers. The matrix material may be a polymer, a metal, or a ceramic. This patent focuses on polymer matrix composites.
The fibers can be made from a variety of materials. These materials include glass, graphite or carbon, polymers, boron, ceramics, or metals. Glass fibers include E-glass (electrical) and S-glass (structural) types. Carbon fibers include those made from different precursors, such as polyacrylonitrile (PAN) or pitch. Polymer fibers include, but are not limited to, aramid (Kevlar(copyright)), polyethylene (Spectra(copyright)), or PBO (Zylon(copyright)). Ceramic fibers may include silicon carbide (SiC) or aluminum oxide (Al2O3).
For cryogenic tanks, the preferred most material is a polymer. The preferred lower is carbon fiber, more preferably PAN-based fibers, more preferably high strength (over 500 ksi) and high modulus (over 30 Msi) fibers. The most preferred fibers are ultra high modulus fibers (over 60 Msi), specifically M55J fiber by Toray.
Another critical parameter for a composite tank is the ability to withstand repeated temperature changes (thermal cycles) without microcracking. One factor that contributes to microcrack resistance is toughness.
The unique, nontraditional concept explored herein is to use hybrid composites to create liquid oxygen tanks. A hybrid composite would allow for oxygen compatible inner layers while have outer layers of a different composite to provide the necessary mechanical properties.
Historically, the approach used to determine the oxygen compatibility of a material was to test the material in the standard mechanical impact test in liquid oxygen (LOX). If the material had an impact threshold of 72 foot-pounds, it was acceptable for use in oxygen environments, such as launch vehicle LOX tanks. If the material""s threshold was less than 72 foot-pounds, it typically was not used. Due to limitations in the testing as well as differences in the material structures between metals and composites, standard high strength graphite reinforced composites typically have not been able to pass at this level when tested at RLV tank wall thicknesses. The approach taken here, which was developed in conjunction with NASA, was to use the standard mechanical impact test to rank composites with respect to each other. Furthermore, an evaluation of the compatibility of composites in oxygen environments would only be determined after testing composite materials with respect to specific ignition mechanisms while in the presence of oxygen. The ignition mechanisms tested included pressurized mechanical impact, particle impact, puncture, puncture of damaged, oxygen-soaked samples, electrostatic discharge, friction, and pyrotechnic shock. If the material could withstand ignition in these environments, it could possibly be considered oxygen compatible.
Oxygen compatibility addresses only one aspect of an oxygen tank for future launch vehicles. The liquid oxygen tank in the Lockheed Martin Michoud Space Systems"" RLV, for example, is the structural backbone for the vehicle. As a result, a composite used for the tank must have outstanding mechanical properties. Unfortunately, the composites with the best structural performance may not necessarily be the composite with the best resistance to ignition, and vice-versa. For example, graphite-reinforced composites provide the best structural performance for a composite tank. However. a composite with an alternate reinforcement, such as glass or Kevlar(copyright), may provide improved oxygen compatibility performance over graphite reinforced composites. Unfortunately, these alternate reinforcements do not provide the high strength-to-weight or strain-to-weight ratios and other structural benefits offered by graphite reinforced composites. Therefore, it is believed that a hybrid material consisting of two different composite systems may provide the optimal mechanical properties and oxygen compatibility. The inner layers of the component, which is in contact with the liquid oxygen, would be made from the more compatible system, while the outer layers of the component would be made from the higher performance system.
The apparatus of the present invention solves the problems confronted in the art in a simple and straightforward manner. What is provided are composite tanks for liquid oxygen.
The composite tanks can be made in accordance with any of the methods disclosed in any reference incorporated herein by reference. The invention could be made, for example, using the method of U.S. Pat. No. 5,403,537.
The interior of the tanks disclosed herein typically have a volume of at least lower, more often at least one gallon, and usually have a volume sufficient to allow their use to contain liquid oxygen for typical aerospace applications.
A mechanical impact of at least 10+ ft-lbs at around xe2x88x92300 degrees F. (at around xe2x88x92350 degrees F. when densified oxygen (slush oxygen) is contained in the tank) is preferred, but is not a requirement. Historically materials have been required to pass the Lox mechanical impact test at 72 ft-lbs or otherwise be shown to be safe for use in application. Lox compatible composite materials have demonstrated the ability to resist combustion when subjected to any potential ignition source. For a composite tank, realistic potential ignition sources were determined to include, but are not limited to mechanisms such as pyrotechnic shock, friction, puncture, electrostatic discharge, and particle impact. These tests are taken to extreme levels. For example, in the puncture test, a sharpened serrated spike pierced a composite material while submerged in Lox without igniting the composite. This same puncture test has ignited Titanium, a material that is not oxygen compatible, and does not ignite aluminum. The pyrotechnic shock test was also an extreme test in that composites were placed in Lox and subjected to shock loads equivalent to existing metallic Lox tanks such as that on the External Tank. The friction test was extreme in that the composites were abraded to dust with a drill bit in pure oxygen without ignition. In the electrostatic discharge test, composites were subjected to energies that could far exceed any instrumentation or static build up; specifically, these composites do not ignite when subjected to 112.5 Joules from 5000 Volts.
The composite feedline of the present invention preferably has an inner diameter of around 1xe2x80x3 to 36xe2x80x3.
As used herein, xe2x80x9cform of fiberxe2x80x9d means uni-directional tape, slit tape, tow, fabric, woven fabric, chopped fiber, or felt.